Grease composition and rolling bearing

ABSTRACT

A grease composition containing base oil, a thickener, and an additive, wherein said additive contains at least one kind selected from among a phosphite ester shown by a chemical formula (1) or a phosphite ester shown by a chemical formula (2); a content of said phosphite ester is 0.5 to 4 parts by weight for 100 parts by weight which is a total of an amount of said base oil and that of said thickener; and said additive contains neither phosphite esters other than said phosphite ester shown by said formula (1) or by said formula (2) nor phosphate esters having organic groups whose carbon number is not less than three:

TECHNICAL FIELD

The present invention relates to a grease composition. Moreparticularly, the present invention relates to a grease compositionuseful for preventing fretting wear (micromotion-caused wear) whichoccurs at sliding-contact portions and connected portions of parts aimedat restraining a relative motion and parts which have a slightreciprocating motion. The present invention also relates to a rollingbearing in which the grease composition is packed. The present inventionparticularly relates to a rolling bearing to be used for motors forindustrial machines and electric equipments and motors for drivingelectric cars, and the like and a rolling bearing to be used byincorporating it in a wind power generator.

BACKGROUND ART

A wear phenomenon (hereinafter referred to as fretting) called afretting wear occurs on various mechanical parts such as parts aimed atrestraining a relative motion of a fit-in portion of a shaft, a boltedportion, a riveted portion, a tapered coupler, and the like; and partssuch as a rolling bearing, a sliding bearing, a ball bush, a splinebearing, a flexible shaft coupling, a universal joint, a constantvelocity joint, a laminated leaf spring, a coil spring, an electriccontact point, a valve, a valve seat, a wire rope, and the like whichhave a slight reciprocating motion. In transporting cars a long distanceby using a trailer, a freight train or the like, the fretting may occuron a rolling surface of the bearing owing to slight vibrations whichoccur during transport. In the case where a bearing is used in alow-temperature environment such as a cold region, lubricating oilcannot be sufficiently supplied to the rolling surface thereof. As aresult, damage such as the fretting is liable to occur.

There is a possibility that the fretting occurs on motors to be used inenvironments in which parts of the motors have slight reciprocatingmotions and vibrations. In motors for industrial machines such as aservo motor, a stepping motor, and the like, there is a fear that thefretting occurs on the rolling surfaces of the bearings due tovibrations of parts thereof which occur when the motors are in operationand slight reciprocating motions of parts thereof. There is a fear thatthe fretting occurs on the rolling surfaces of the bearings of motorsfor electric equipments for use in cars and on the rolling surfaces ofthe bearings of motors for driving electric cars and hybrid cars owingto slight vibrations caused by an engine power and the like and owing toslight vibrations caused by circumstances of road surfaces while carsare traveling. In the case where lubricating oil is not sufficientlysupplied to the rolling surfaces of the bearings in low-temperatureenvironments, the fretting is liable to occur.

In conformity to a tendency that a motor is miniaturized in recentyears, the miniaturization of a bearing for supporting the rotor of themotor is being advanced. Therefore there is tendency that a high contactsurface pressure is applied to members constructing the bearing for themotor. Further there is a large increase in the rotational speed of themotor. Thus when the operation of the motor is started and stopped,there is tendency that the bearing for the motor has a high accelerationand deceleration in the rotation of the motor. Owing to an increase in asurface pressure at a portion between rolling elements and a bearingring and an increase in sliding at the above-described portion caused bya sudden acceleration and a sudden deceleration in the rotation of thebearing for the motor, an oil film is liable to break (defectivelubrication) at the above-described portion. Thereby metal contactoccurs and thus the rolling element and the bearing ring are liable towear, which increases the degree of fear of the occurrence of thefretting.

A rolling bearing and particularly a large double-row roller bearing asshown in FIG. 5 are frequently used as a bearing supporting a main shaftof a large wind power generator. A rotor blade 32 is mounted on a rotorshaft (main shaft) 33. The rotor shaft 33 is rotated by a wind powerreceived thereby. The rotation of the rotor shaft 33 is increased by aspeed-up gear 36 to rotate a power generator 37 and generate an electricpower. While the electric power is being generated by the received windpower, an axial load (axial load applied to bearing) and a radial load(radial load applied to bearing) generated by a wind power applied tothe rotor blade 32 are applied to the rotor shaft 33 supporting therotor blade 32. A blade bearing for use in a blade pitch pivotal seatand a yaw bearing for use in a yaw pivotal seat are used for the windpower generator. To receive wind efficiently, the blade bearing ismounted on the base of a blade to rotatably support the blade so thatthe angle of the blade can be adjusted in dependence on the strength ofthe wind. The yaw bearing pivotally supports the yaw of a nacelle toadjust the direction of a main shaft according to the direction of wind.

Because the main shaft-supporting bearing, the blade bearing, and theyaw bearing are operated in environments where the direction and powerof wind always fluctuate, the main shaft-supporting bearing, the bladebearing, and the yaw bearing are repeatingly oscillated in oppositedirections in dependence on the direction of the wind and slowly andquickly by the strength of the wind. As a result, the fretting may occuron the rolling surface of the bearing. The main shaft-supporting bearingirregularly repeats start, acceleration, deceleration, stop with themain shaft-supporting bearing being always subjected to a loadfluctuation. In addition, because vibrations of the blade and a gear boxare applied to the main shaft-supporting bearing. Thus there is a highpossibility that the fretting occurs on the main shaft-supportingbearing. In the wind power generator which is installed on the top of amountain and in a very cold district and is thus subjected to very lowtemperatures, grease hardens and thus lubricating oil cannot besufficiently supplied to the rolling surface of the bearing. Thereforethere is a fear that the fretting has a large damage.

Various methods for preventing the occurrence of the fretting have beenproposed. In one of these proposed methods, an appropriate lubricant isselected to prevent the occurrence of the fretting. In this method, atleast one substance selected from among oxidized paraffin,diphenylhydrogen phosphite, and hexamethylphosphoric triamide is addedto the urea-based thickener (see patent document 1).

In a hub bearing for rotatably supporting wheels of a car, carbon steelfor machine structural use such as S53C which has a high forgeabilityand is inexpensive is used in a second generation hub bearing (GEN 2)and a third-generation hub bearing (GEN 3) provided with a flange on theouter ring thereof. In the hub bearing composed of the carbon steel formachine structural use, by subjecting the raceway part to high frequencyheat treatment, the rolling fatigue strength of the bearing part issecured. But the carbon steel for machine structural use contains asmall amount of the alloy component and thus has a weak surfacestrength. Therefore the carbon steel for machine structural use isinferior to bearing steel in a resistance to peeling of grease arisingfrom the surface of the bearing part. As a measure for solving theabove-described problem of the carbon steel for machine structural use,grease containing zinc dithiocarbamate added to its base oil as itsessential component is known (see patent document 2).

As grease for preventing the fretting from occurring in the hub bearingor the like in a wide temperature range from low to high temperatures byusing base oil comparatively inexpensive, the grease containing thepredetermined base oil, the poly(meta)acrylate, zinc dithiophosphate,and the phosphate is proposed (see patent document 3).

PRIOR-ART TECHNICAL DOCUMENT Patent Document

Patent document 1: U.S. Pat. No. 2,576,898

Patent document 2: Japanese Patent Application Laid-Open No. 2006-342260

Patent document 3: Japanese Patent Application Laid-Open No. 2010-84142

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

The grease of the patent document 1, however, does not have a sufficientperformance in preventing the occurrence of the fretting at lowtemperatures. The grease of the patent document 2 is also insufficientin the resistance to the peeling thereof arising from the surface of abearing. Thus in the case of the bearing and the like for which thegrease of the patent document 2 is used, the fretting is liable to occurat low temperatures.

It is difficult for the grease of the patent document 1 to sufficientlyprevent the occurrence of the fretting in the case where the grease ofthe patent document 1 is used for motors in which parts thereof areliable to have slight reciprocating motions and vibrations. It is alsodifficult for the grease of the patent document 1 to sufficientlyprevent the occurrence of the fretting in the case where the grease isused for the rolling bearing which is used by incorporating it n thewind power generator where the direction and power of wind alwaysfluctuate and which is repeatingly oscillated in opposite directions independence on the direction of the wind and slowly and quickly by thestrength of the wind.

In the present situation, the effect of improving the fretting wearresistance to be achieved by the grease used for lubrication has notbeen sufficiently clarified. For example, there is a case in whichgrease containing the same thickener gives contradictory results in thefretting wear resistance in dependence on test methods. There are manyreports that greases containing phosphorous compounds such as aphosphate or a phosphate ester as the additive thereof are preferable.But the fretting wear resistances of the greases are greatly differentfrom one another according to the structures of the phosphorouscompounds.

The present invention has been made to deal with the above-describedproblems. Therefore it is an object of the present invention to providea grease composition having an excellent fretting wear resistance in awide range from low to high temperatures and particularly even at lowtemperatures and a rolling bearing in which the grease composition ispacked. It is another object of the present invention to provide arolling bearing which has an excellent fretting wear resistance and iscapable of preventing the rolling surface thereof from wearing in thecase where the rolling bearing is used for a motor and a wind powergenerator.

Means for Solving the Problem

The grease composition of the present invention contains base oil, athickener, and an additive. The additive contains at least one kindselected from among a phosphite ester shown by a chemical formula (1) ora phosphite ester shown by a chemical formula (2). The content of thephosphite ester is 0.5 to 4 parts by weight for 100 parts by weightwhich is a total of an amount of the base oil and that of the thickener.

(In the chemical formulas (1) and (2), R₁, R₂, R₃, R₄, and R₅ denote amethyl group or an ethyl group. R₁ and R₂ are the same group ordifferent groups. R₃, R₄, and R₅ are the same group or differentgroups).

The grease composition contains neither phosphite esters other than thephosphite ester shown by the formula (1) or by the formula (2) norphosphate esters having organic groups whose carbon number is not lessthan three.

The phosphite ester is dimethyl phosphite, diethyl phosphite, trimethylphosphite or triethyl phosphite.

The thickener is a urea compound shown by a chemical formula (3):

(In the chemical formula (3), R₇ denotes a divalent hydrocarbon groupwhose carbon number is 6 to 15. R₆ and R₈ denote monovalent hydrocarbongroups each containing an aromatic ring. R₆ and R₈ are the same group ordifferent groups.)

The base oil is mineral oil and/or synthetic oil. The kinetic viscosityof the base oil at 40° C. is 20 to 150 mm²/s. The worked penetration ofthe grease composition is 200 to 350.

A rolling bearing of the first aspect of the present invention has aninner ring; an outer ring; and a plurality of rolling elementsinterposed between the inner ring and the outer ring. The greasecomposition of the present invention is packed inside the rollingbearing such that the grease composition is applied to circumferences ofthe rolling elements. The rolling bearing is used as a hub bearing.

A rolling bearing of the second aspect of the present invention supportsa rotor of a motor. The rolling bearing has an inner ring; an outerring; a plurality of rolling elements disposed between the inner ringand the outer ring; and a sealing member provided at openings disposedat both axial ends of the inner ring and the outer ring. The greasecomposition of the present invention is packed inside the rollingbearing such that the grease composition is applied to circumferences ofthe rolling elements.

A rolling bearing of the third aspect of the present invention isincorporated in a wind power generator. The rolling bearing has aplurality of rolling elements interposed between an inner ring and anouter ring such that the rolling elements are capable of rolling. Thegrease composition of the present invention is packed inside a space,formed between the inner ring and the outer ring, in which the rollingelements are disposed. In the third aspect, the base oil is syntheticoil. The kinetic viscosity of the base oil at 40° C. is 300 to 600mm²/s.

Effect of the Invention

Because the grease composition of the present invention contains apredetermined amount of the phosphite ester shown by the chemicalformula (1) or (2) as the additive, the grease composition is capable ofpreventing the occurrence of the fretting and thus has an excellentfretting wear resistance. In addition, because the predetermined diureais used as the thickener of the grease composition, the greasecomposition is capable of preventing the occurrence of the fretting in awide temperature region from low to high temperatures. Therefore thefield of industry for which the grease composition is utilizable is verywide and thus it is possible to use the grease composition of thepresent invention for various equipments.

The above-described grease composition is packed inside the rollingbearing of the present invention, the rolling bearing is allowed toprevent the occurrence of the fretting in a wide temperature region fromlow to high temperatures. Therefore the rolling bearing can be used as ahub bearing for rotatably supporting wheels of a car.

Although the rolling bearing of the present invention is used inenvironments in which parts thereof have slight reciprocating motionsand vibrations, the rolling bearing has an excellent fretting wearresistance and is thus allowed to prevent the fretting from occurring onthe rolling surface thereof in a wide temperature region. Therefore therolling bearing of the present invention can be preferably utilized tosupport rotors of motors for industrial machines and electric equipmentsand rotors of motors for driving electric cars.

Although the rolling bearing of the present invention is used inenvironments in which parts thereof are repeatingly oscillated, therolling bearing has an excellent fretting wear resistance and is thusallowed to prevent the fretting from occurring on the rolling surfacethereof in a wide temperature region. Therefore the rolling bearing ofthe present invention can be preferably utilized to support the mainshaft, blade, and yaw of a wind power generator.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a sectional view showing a deep groove ball bearing as anexample of a rolling bearing of the present invention.

FIG. 2 is a sectional view showing a hub bearing as another example ofthe rolling bearing of the present invention.

FIG. 3 is a sectional view of a motor in which the rolling bearing ofthe present invention is used.

FIG. 4 is a schematic view of an entire wind power generator in whichthe rolling bearing of the present invention is used.

FIG. 5 shows details of the construction of a main shaft-supportingportion shown in FIG. 4.

FIG. 6 is a sectional view showing a rolling bearing for supporting themain shaft of the wind power generator.

MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention are described in detailbelow. The grease composition of the present invention contains baseoil, a thickener, and an additive. The additive contains predeterminedphosphite ester.

The predetermined phosphite ester to be used for the grease compositionof the present invention is at least one kind selected from amongphosphite ester shown by a chemical formula (1) or phosphite ester shownby a chemical formula (2). One of the phosphite esters shown by theformulas (1) and (2) may be used singly or in combination of not lessthan two kinds.

(In the chemical formulas (1) and (2), R₁, R₂, R₃, R₄, and R₅ denote amethyl group or an ethyl group. R₁ and R₂ are the same group ordifferent groups. R₃, R₄, and R₅ are the same group or differentgroups).

The phosphite ester shown by the formula (1) or the formula (2) includesphosphite diester such as dimethylphosphite, diethyl phosphite, andmethyl ethyl phosphite; and phosphite triester such as trimethylphosphite, triethyl phosphite, dimethyl ethyl phosphite, and methyldiethyl phosphite. Of these phosphite esters, the dimethyl phosphite,the diethyl phosphite, the trimethyl phosphite, and the triethylphosphite are preferable.

In the grease composition of the present invention, the content of thephosphite ester for 100 parts by weight which is the total of the amountof the base oil and that of the thickener is 0.5 to 4 parts by weight.When the content of the phosphite ester is less than 0.5 parts byweight, there is a possibility that the fretting wear resistance of thegrease composition cannot be improved. When the content of the phosphiteester is more than four parts by weight, it is difficult to improve thefretting wear resistance of the grease composition to a higher extent.In the case where not less than two kinds of the phosphite esters areused, the total of the amounts thereof is set to the above-describedrange as the content of the phosphite ester.

It is preferable that the grease composition of the present inventiondoes not contain phosphite esters other than the phosphite ester shownby the formula (1) or shown by the formula (2). It is also preferablethat the grease composition of the present invention does not containphosphate esters having organic groups whose carbon number is not lessthan three. That is, it is also preferable that the grease compositionof the present invention does not contain substances other than thoseobtained by oxidizing the phosphite ester shown by the formula (1) orshown by the formula (2). In the case where the grease composition ofthe present invention contains the above-described unpreferablephosphite esters or the phosphate esters, there is a fear that thegrease composition is inferior in its fretting resistance. There is afear that the grease composition is inferior in its fretting resistanceat low temperatures.

By using the predetermined phosphite ester having a low molecular weightas the additive of the grease composition of the present invention, thegrease composition is capable of preventing the occurrence of thefretting without using other phosphorous compounds such as zincdithiophosphate, tricresyl phosphate, and the like and other extremepressure agents which have been hitherto used to prevent the frettingfrom occurring and the grease from peeling from the surface of thebearing.

The grease composition of the present invention may contain knownadditives other than the above-described phosphite esters as necessary.As such additives, phenol-based and amine-based antioxidants; arustproof agent consisting of carboxylates, sulfonates or the like; anabrasion-resistant agent consisting of polyalkylene glycol, glycerin orthe like; the extreme pressure agent such as chlorinated paraffin,sulfurized oil, organic molybdenum compounds, and the like; an oilinessimprover such as higher fatty acid, synthetic ester, and the like; and asolid lubricating agent such as graphite, molybdenum disulfide or thelike are listed. These additives can be added to the base oil singly orin combination of not less than two kinds.

The base oil of the grease composition of the present invention is notlimited to a specific one, but it is possible to use those normally usedin the field of grease. For example, mineral oil, synthetic oil, andmixed oils thereof are listed.

As the mineral oil, those to be obtained by methods normally carried outin the process of producing lubricating oil in the petroleum refiningindustry are listed. More specifically, the mineral oil which is refinedby performing not less than one of the following processings of alubricating oil distillate obtained by distilling crude oil at a normalpressure and at a decreased pressure: The processings include solventdeasphalting, solvent extraction, hydrogenolysis, solvent dewaxing,catalytic dewaxing, hydrogenation refining, sulfuric acid cleaning, andclay treatment.

As the above-described synthetic oil, ester oil, polybutene,poly-α-olefin (hereinafter referred to as PAO oil) such as 1-octeneoligomer, 1-decene oligomer, and the like or hydrides thereof; alkylnaphthalene; alkyl benzene; polyoxyalkylene glycol; polyphenyl ether;dialkyl diphenyl ether; silicone oil; fluorine oil; and GTL oilsynthesized by Fischer-Tropsh process are listed.

In packing the grease composition in a rolling bearing for a motor, itis preferable to use at least one oil selected from among the ester oiland the PAO oil of these synthetic oils, because the ester oil and thePAO oil are excellent in the heat resistance and lubricating propertythereof. It is especially preferable to essentially use the ester oil.It is preferable to use a mixed oil of the ester oil and the PAO oil asnecessary. In using the mixed oil of the ester oil and the PAO oil, itis preferable to set the mixing ratio of the PAO oil to the ester oil(ratio by weight) to 8 to 2 to 2 to 8.

The ester oil is a compound which has an ester group in its molecule andis liquid at a room temperature. As the ester oil, polyol ester oil,phosphate ester oil, polymer ester oil, aromatic ester oil, carbonateester oil, and diester oil are listed. Of these ester oils, the aromaticester oil and the polyol ester oil are preferable.

The aromatic ester oil to be obtained by a reaction between an aromaticpolybasic acid or a derivative thereof and a higher alcohol. As thearomatic polybasic acid, tricarboxylic acid such as trimellitic acid,biphenyl tricarboxylic acid, and naphthalene tricarboxylic acid;aromatic tetracarboxylic acid such as pyromellitic acid, biphenyltetracarboxylic acid, benzophenone tetracarboxylic acid, and naphthalenetetracarboxylic acid; and derivatives such as acid anhydrides thereofare listed. As the higher alcohol, aliphatic monovalent alcohols whosecarbon number is not less than four such as octyl alcohol, decylalcohol, and the like are preferable. As examples of the aromatic esteroil, trioctyl trimellitate, tridecyl trimellitate, and tetraoctylpyromellitate are listed.

As the polyol ester oil, compounds having a plurality of ester groups inthe molecule to be obtained by the reaction between polyol and amonobasic acid are preferable. The monobasic acid to be reacted with thepolyol may be used singly or as a mixture. Dibasic acids maybe used inthe case of oligoester. As the polyol, trimethylolpropane,pentaerythritol, dipentaerythritol, neopentyl glycol,2-methyl-2-propyl-1, and 3-propanediol are listed. As the monobasicacid, monovalent aliphatic acids having carbon numbers 4 through 18 arelisted. For example, valeric acid, caproic acid, caprilic acid, enanthicacid, pelargonic acid, capric acid, undecanoic acid, lauric acid,myristic acid, palmitic acid, stearic acid, caproleic acid, undecylenicacid, linderic acid, tsuzuic acid, physeteric acid, myristoleic acid,palmitoleic acid, petroselinic acid, oleic acid, elaidic acid,asclepinic acid, vaccenic acid, sorbic acid, linoleic acid, sabinicacid, and ricinoleic acid are listed. As the polyolester oil,trimethylolpropane caprylate, trimethylolpropane pelargonate,pentaerythritol-2-ethylhexanoate, pentaerythritol pelargonate arelisted.

As examples of the diester oil, ditridecyl glutarate, di-2-ethylhexyladipate, di-isodecyl adipate, ditridecyl adipate, and di-3-ethylhexylsebacate are listed.

In packing the grease composition in a rolling bearing for a wind powergenerator, the synthetic oil is mainly used as the base oil of thegrease composition. Of the above-described synthetic oils, it ispreferable to use the PAO oil because the PAO oil is excellent in itsoxidation stability and the performance at low temperatures (rollingbearing is frequently used in cold places such as highland).

The kinetic viscosity of the base oil at 40° C. is preferably 20 to 150mm²/s. When the kinetic viscosity of the base oil at 40° C. is less than20 mm²/s, the viscosity is so low that an oil film is liable to break,and the oil vaporizes in a large amount. On the other hand, when thekinetic viscosity of the base oil at 40° C. is more than 150 mm²/s, theperformance of supplying the lubricating oil to a rolling surface of therolling bearing at low temperatures is inferior and thus the fretting isliable to occur. In addition power loss becomes large. Thus in the casewhere the grease composition is used for the bearing, the torque thereofrises and the calorific value becomes high. In the case where the mixedoil is used as the base oil of the grease composition, it is preferablethat the kinetic viscosity of the mixed oil falls within theabove-described range.

In packing the grease composition in the rolling bearing for the motor,the kinetic viscosity of the base oil at 40° C. is favorably 20 to 50mm²/s, more favorably 20 to 40 mm²/s, and most favorably 26 to 40 mm²/s.When the kinetic viscosity of the base oil at 40° C. is more than 50mm²/s, the performance of supplying the lubricating oil to the rollingsurface is inferior. Thus the fretting is liable to occur when the motoris used in environments in which parts thereof have slight reciprocatingmotions and vibrations. In addition because the torque of the rollingbearing rises, power loss becomes large when the motor is driven and thecalorific value also becomes large.

In packing the grease composition in the rolling bearing for the windpower generator, the kinetic viscosity of the base oil at 40° C. isfavorably 300 to 600 mm²/s. The kinetic viscosity of the base oil at 40°C. is more favorably 350 to 550 mm²/s and most favorably 400 to 500mm²/s. The wind power generator is so constructed that a main shaft onwhich a blade is mounted is rotated by a wind power. When the wind poweris low, the main shaft rotates at a very low number of rotations. Thusthe oil film of the bearing supporting the main shaft becomescomparatively thin and thus unstable. Therefore when the kineticviscosity of the base oil at 40° C. is less than 300 mm²/s, there is afear that the oil film is liable to break. On the other hand, when thekinetic viscosity of the base oil at 40° C. is more than 600 mm²/s, theperformance of supplying the lubricating oil to the rolling surface ofthe rolling bearing is inferior and thus the fretting is liable tooccur. In addition the torque of the bearing rises and thus there is afear that power generation efficiency deteriorates.

The thickener of the grease composition of the present invention is notlimited to a specific one, but it is possible to use those normally usedin the field of grease. For example, it is possible to use soap-basedthickeners such as metal soap and complex metal soap; and non-soap-basedthickeners such as Benton, silica gel, urea compounds, and urea•urethane compounds. As the metal soap, sodium soap, calcium soap,aluminum soap, and lithium soap are listed. As the urea compounds andthe urea• urethane compounds, diurea compounds, triurea compounds,tetraurea compounds, other polyurea compounds, and diurethane compoundsare listed. Of these thickeners, it is preferable to use the lithiumsoap (lithium complex soap is included) or the urea compound, becausethe lithium soap and the urea compound are excellent in the heatresistance thereof and in the performance of entering into and adheringto the rolling surface and thus easily capable of preventing theoccurrence of the fretting. As the urea compound, a diurea compoundshown by the following chemical formula (3) is preferable.

In the chemical formula (3), R₆ and R₈ denote monovalent hydrocarbongroups each containing an aromatic ring. R₆ and R₈ may be the same groupor different groups. Because each of R₆ and R₈ consists of themonovalent hydrocarbon group containing the aromatic ring, the greasecomposition of the present invention containing the diurea compound isexcellent in its heat resistance, durability, and fretting wearresistance. As such groups, listed are a phenyl group, a naphthyl group,an alkyl aryl group in which one or a plurality of hydrogen atoms of thephenyl group or the naphthyl group are substituted with one or aplurality of alkyl groups, and an aryl alkyl group in which one or aplurality of hydrogen atoms of the alkyl group is substituted with anaryl group such as the phenyl group or the naphthyl group.

Although the carbon number of an aromatic hydrocarbon group of R₆ andthat of R₈ is not specifically limited respectively, the aromatichydrocarbon group whose carbon number is 6 to 15 is preferably used. Asthe hydrocarbon group, whose carbon number is 6 to 15, containing thearomatic ring, a toluic group, a xyl group, a β-phencyl group, at-butylphenyl group, a dodecylphenyl group, a benzyl group, andmethylbenzyl group are listed.

In the chemical formula (3), R₇ denotes a divalent hydrocarbon groupwhose carbon number is 6 to 15. As such a hydrocarbon group, astraight-chain or a branched chain alkylene group, a straight-chain or abranched chain alkenylene group, a cycloalkylene group, and an aromaticgroup are listed.

The urea compound is obtained by allowing isocyanate and monoamine toreact with each other. To prevent a reactive free radical from beingleft, it is preferable to allow the isocyanate group of the isocyanatecompound and the amino group of the amine compound to react with eachother in almost an equivalent amount.

The diurea compound shown by the chemical formula (3) is obtained by areaction between diisocyanate and monoamine. As the diisocyanate,phenylene diisocyanate, diphenyl diisocyanate, diphenylmethanediisocyanate, 1,5-naphtylene diisocyanate, 2,4-tolylene diisocyanate,3,3-dimethyl-4,4-biphenylene diisocyanate, octadecane diisocyanate,decane diisocyanate, and hexane diisocyanate are listed. As themonoamine, aniline and p-toluidine are listed.

The polyurea compound is obtained by allowing the diisocyanate and themonoamine as well as the diamine to react with each other. Thediisocyanate and the monoamine similar to those used to form the diureacompound are used. As the diamine, ethylenediamine, propanediamine,butanediamine, hexanediamine, octanediamine, phenylenediamine,tolylenediamine, xylenediamine, and diaminodiphenylmethane are listed.

In enclosing the grease composition in the rolling bearing for a motor,because the rolling bearing is demanded to have quietness and a lowtorque, of the urea compounds, it is preferable to use an aliphatic ureacompound to be obtained by allowing the diisocyanate such asdiphenylmethane diisocyanate and aliphatic monoamine such as octylamineto react with each other.

The base grease to which the above-described additive (phosphite ester)is to be added is obtained by adding the thickener such as the lithiumsoap or the urea compound to the base oil. The base grease containingthe urea compound as its thickener is produced by allowing thepolyisocyanate component and the monoamine components to react with eachother in the base oil.

The content of the thickener which occupies in 100 parts by weight ofthe base grease is favorably 3 to 40 parts by weight and more favorably5 to 30 parts by weight. The content of the thickener in this range issuitable for obtaining the intrinsic lubricating property of the greasecomposition. When the content of the thickener is less than three partsby weight, the thickener has a small thickening effect and thus it isdifficult to form the grease composition. When the content of thethickener is more than 40 parts by weight, obtained base grease is sohard that it is difficult to obtain the desired effect.

It is preferable that the worked penetration (JIS K 2220) of the greasecomposition of the present invention is in the range of 200 to 350. Whenthe worked penetration thereof is less than 200, the grease compositionhas a low degree of oil separation at low temperatures and a defectivelubrication. Thereby the fretting is liable to occur. On the other hand,when the worked penetration of the grease composition is more than 350,obtained grease is soft and liable to flow outside the bearing, which isunpreferable.

The grease composition of the present invention is excellent forpreventing the occurrence of the fretting wear (micromotion-causedwear). Therefore the grease composition is useful as grease for variousmechanical parts such as parts aimed at restraining a relative motion ofa fit-in portion of a shaft, a bolted portion, a riveted portion, atapered coupler, and the like; and parts such as a rolling bearing, asliding bearing, a ball bush, a spline bearing, a flexible shaftcoupling, a universal joint, a constant velocity joint, a laminated leafspring, a coil spring, an electric contact point, a valve, a valve seat,a wire rope, and the like which have a slight reciprocating motion.

FIG. 1 shows an example of the rolling bearing of the present invention.FIG. 1 is a sectional view of a deep groove ball bearing in which thegrease composition is packed. In the deep groove ball bearing 1, aninner ring 2 having an inner ring rolling surface 2 a on its peripheralsurface and an outer ring 3 having an outer ring rolling surface 3 a onits inner peripheral surface are concentrically disposed, and aplurality of rolling elements 4 is disposed between the inner ringrolling surface 2 a and the outer ring rolling surface 3 a. A cage 5holds a plurality of the rolling elements 4. A sealing member 6 fixed tothe outer ring 3 is provided at openings 8 a and 8 b disposed at bothaxial ends of the inner ring 2 and the outer ring 3. A greasecomposition 7 of the present invention is packed inside the deep grooveball bearing 1 such that the grease composition is applied to at leastthe circumferences of the rolling elements 4.

FIG. 2 shows an example (third-generation hub bearing for use in drivenwheel) in which the rolling bearing of the present invention is used asa hub bearing. FIG. 2 is a sectional view showing the hub bearing. A hubbearing 16 has an inner ring (called an inward member) 15 having a hubring 11 and a driving inner ring 12 and an outer ring (called an outwardmember) 13, and double-row rolling elements 14, 14. The hub ring 11 hasa wheel-mounting flange 11 d, integral therewith at its one end, onwhich a wheel (not shown) is mounted. Formed on the outer periphery ofthe hub ring 11 are an inside rolling surface 11 a and a small-diameterstepped portion 11 b extended axially from the inside rolling surface 11a. In the present specification, “outside” in the axial direction meansan outer side in the widthwise direction in a state where the hubbearing 16 is mounted on a car, whereas “inside” means a central side inthe widthwise direction. The driving inner ring 12 having an insiderolling surface 12 a formed on the outer periphery thereof is fitted inthe small-diameter stepped portion lib of the hub ring 11 by press fit.The removal of the inner ring 12 is prevented from being axially removedfrom the hub ring 11 by a caulking portion 11 c formed by plasticallydeforming an end of the small-diameter stepped portion 11 b of the hubring 11 radially outward. The outer ring 13 has on its outer periphery acar body-mounting flange 13 b on which the car body is mounted and whichis integral with the outer ring 13 and has on its inner periphery theoutside rolling surfaces 13 a, 13 a. The double-row rolling elements 14,14 are accommodated between the outside rolling surfaces 13 a, 13 a andthe inside rolling surfaces 11 a, 12 a opposed to the double-row outsiderolling surfaces 13 a, 13 a such that the double-row rolling elements14, 14 are capable of rolling. The grease of the present invention ispacked in a space surrounded with a sealing member 17, the outer ring13, a sealing member 18, the inner ring 15, and the hub ring 11 so thatthe grease covers the circumference of the double-row rolling elements14, 14 sandwiched between the outer ring 13 and the inner ring 15 andlubricates a rolling contact portion between the rolling surfaces of therolling elements 14, 14 and the inside rolling surfaces 11 a, 12 a aswell as the outside rolling surfaces 13 a, 13 a. Because the greasecomposition of the present invention is packed in the hub bearing 16,the fretting can be prevented from occurring in a wide temperatureregion from low to high temperatures.

As materials that can be used for the above-described hub bearing, it ispossible to list bearing steel, carburized steel, and carbon steel formachine structural use. Of these materials, it is preferable to use thecarbon steel for machine structural use such as S53C which has highforgeability and is inexpensive. The carbon steel for machine structuraluse is used after the rolling fatigue strength of a bearing portion issecured by heat-treating it with a high-frequency wave.

FIG. 3 shows an example of a motor for which the rolling bearing of thepresent invention is used. FIG. 3 is a sectional view of theconstruction of the motor. The motor has a stator 29 consisting of amagnet, for the motor, disposed on an inner peripheral wall of a jacket28, a rotor 23 around which a winding 22 fixed to a rotating shaft 21 iswound, a commutator 24 fixed to the rotating shaft 21, a brush holder 25disposed on an end frame 27 supported by the jacket 28, and a brush 26accommodated inside the brush holder 25. The rotating shaft 21 isrotatably supported on the jacket 28 by the deep groove ball bearing 1(see FIG. 1) of the present invention and a supporting construction forthe bearing 1. Because the grease composition of the present inventionis packed inside the bearing 1, the bearing 1 has an excellent frettingwear resistance, although it is used in environments in which partsthereof have slight reciprocating motions and vibrations. Therefore thebearing 1 is capable of preventing the fretting from occurring on therolling surface thereof in a wide temperature region.

As the rolling bearing for the motor, in addition to the deep grooveball bearing shown in FIG. 1, it is possible to use an angular ballbearing, a cylindrical roller bearing, a tapered roller bearing, aself-aligning roller bearing, a needle-shaped roller bearing, a thrustcylindrical roller bearing, a thrust tapered roller bearing, a thrustneedle-shaped roller bearing, and a thrust self-aligning roller bearing.Of these rolling bearings, it is preferable to use the deep groove ballbearing having preferable rotation accuracy at a high-speed rotation, ahigh load resistance, and a low cost.

As motors to which the rolling bearing of the present invention isapplicable, motors for industrial machines such as a motor for aventilation fan, a blower motor for a fuel battery, a cleaner motor, afan motor, a servo motor, a stepping motor; motors for electricequipments such as a starter motor of a car, an electromotive powersteering motor, a steering-adjusting tilt motor, a blower motor, a wipermotor, and a power window motor; and motors for driving electric cars,hybrid cars, and the like. The rolling bearing of the present inventionis applicable to other motors to be used in environments in which partsthereof have slight reciprocating motions and vibrations.

A wind power generator using the rolling bearing of the presentinvention is described with reference to FIGS. 4 and 5. FIG. 4 is anillustration of the entire wind power generator. FIG. 5 shows thedetails of a portion supporting a main shaft shown in FIG. 4. As shownin FIG. 4, a wind power generator 31 has a rotor blade 32, a generator37 to be driven through a speed-up gear 36 mounted on a rotor shaft(main shaft) 33 of the rotor blade 32, the speed-up gear 36, and a tower38 holding a nacelle 34 accommodating the generator 37 at apredetermined level. The generator 37 is driven by the rotation of therotor blade 32. A power generated by the generator 37 is taken out as anelectric energy. As shown in FIG. 5, the rotor shaft 33 on which therotor blade 32 is mounted is rotatably supported by a bearing 35 mountedin a bearing housing 45 disposed inside the nacelle 34. The bearing 35is the rolling bearing of the present invention. Two bearings 35 areprovided in the example shown in FIG. 5, but one bearing 35 may beprovided.

The speed-up gear 36 increases the rotational speed of the rotor shaft33 and transmits an increased rotational speed to an input shaft of thegenerator 37. The nacelle 34 is pivotally mounted on the tower 38through a yaw pivotal seat bearing 47 and pivoted by a motor 39 of FIG.5 for pivotal use through a reducer 40. The nacelle 34 is pivoted toadjust the direction of the rotor blade 32. The rolling bearing (notshown in the drawings) is mounted on a blade pitch pivotal seat disposedat a base of the rotor blade 32.

The main shaft-supporting bearing 35 is described below with referenceto FIG. 6. FIG. 6 is a sectional view showing the main shaft-supportingbearing which is one example of the rolling bearing of the presentinvention. The bearing 35 is a double-row self-aligning roller bearing.The bearing 35 has an inner ring 41 and an outer ring 42 both of whichmake a pair of bearing rings and a plurality of rolling elements 43interposed between the inner ring 41 and the outer ring 42. A raceway 42a of the outer ring 42 of the bearing 35 is spherical. The outerperipheral surface of each rolling element 43 is formed as a rollerspherical along the raceway 42 a. The inner ring 41 has a flangedconstruction having raceway 41 a, 41 a of respective rows separately.The rolling elements 43 of the respective rows are held by cages 44respectively. The outer ring 42 is mounted on the bearing housing 45 byfitting the outer ring 42 on the inside-diameter surface thereof. Theinner ring 41 is fitted on the outer periphery of the rotor shaft 33 andsupports the rotor shaft 33. The bearing housing 45 has a seal 46 suchas a labyrinth seal between a side wall portion 45 a covering both endsof the bearing 35 and the rotor shaft 33. Because the bearing housing 45provides a sealing property, a construction not having a seal is usedfor the bearing 35. The grease composition of the present invention ispacked inside a space, formed between the inner ring 41 and the outerring 42, in which the rolling elements 43 are disposed. Because thegrease composition of the present invention is packed inside the bearing35, the bearing 35 is allowed to have an excellent fretting wearresistance, although the bearing 35 is used in environments in whichparts thereof are repeatingly oscillated and is thus capable ofpreventing the fretting from occurring on the rolling surface thereof ina wide temperature region.

As the rolling bearing for the wind power generator, in addition to therolling bearing shown in FIG. 6, it is possible to use the deep grooveball bearing shown in FIG. 1, the angular ball bearing, the cylindricalroller bearing, the tapered roller bearing, the needle-shaped rollerbearing, the thrust cylindrical roller bearing, the thrust taperedroller bearing, the thrust needle-shaped roller bearing, and the thrustself-aligning roller bearing.

As the main shaft-supporting bearing 35, it is possible to use radialbearings to which a thrust load can be applied. In addition to theself-aligning roller bearing shown in FIG. 6, of the above-describedrolling bearings, it is possible to use the angular ball bearing, thetapered roller bearing, and the deep groove ball bearing as the mainshaft-supporting bearing 35. Of these rolling bearings, as the mainshaft-supporting bearing of the wind power generator which is operatedin a state in which the main shaft is subjected to a wide range of aload from a light load to a heavy load which is applied thereto when agust of wind blows and in which the direction of wind always change, theself-aligning roller bearing capable of absorbing the operation-causedflexure of the main shaft is especially preferable. As a blade bearingand a yaw bearing, the deep groove ball bearing and the angular ballbearing are preferable.

EXAMPLES

Although the present invention is described more specifically below byway of examples and comparative examples, the present invention is notrestricted thereby.

Examples A-1 Through A-6 and Comparative Examples A-1 Through A-6

As shown in table 1, base grease was prepared by using aromatic urea asits thickener and mineral oil (TURBINE OIL 68 produced by JX Nippon Oil& Energy Corporation) as its base oil. P-toluidine (produced by WakoPure Chemical Industries, Ltd.) and MDI (MILLIONATE MT (main component:diphenylmethane diisocyanate) produced by Nippon Polyurethane IndustryCo., Ltd.) were used as the material of the thickener. The method ofpreparing the base grease (200 g scale (amount of thickener: 18 wt %))was carried out as shown below.

(1) After 16.8 g of the p-toluidine and 119.4 g of the MDI were weighedand put in separate beakers, 82 g of the TURBINE OIL 68 serving as thebase oil was added to the p-toluidine and the MDI. (2) Each mixture washeated up to about 80° C. by using a mantle heater to dissolve thep-toluidine and the MDI in the base oil respectively. (3) The base oilin which the p-toluidine was dissolved was added to the base oil inwhich the MDI was dissolved with the base oil being stirred to formgrease. At this time, the temperature was set to 100 to 120° C. (4) Byusing an infrared spectroscopy (IR), the band of isocyanate (N═C═O) inthe neighborhood of 2280 cm⁻¹ was checked. (5) The mixture was put in aconstant-temperature bath having a temperature of 130° C. for two hours(stirred every 30 minutes) to promote a reaction between the p-toluidineand the MDI. Based on the band of the isocyanate (N═C═O) in theneighborhood of 2280 cm⁻¹, whether the reaction proceeded was checked.The mixture was naturally cooled overnight at a room temperature. (6)The mixture was applied to a triple roll to obtain the base grease.

After an additive was added to 100 parts by weight of each of the basegreases prepared in the above-described manner at a mixing ratio shownin table 1, each of the mixtures was sufficiently stirred by astirring/defoaming machine to obtain a specimen grease of each of theexamples and the comparative examples. A low-temperature fretting testshown below was conducted on the obtained specimen greases to measurethe wear amount of each of the specimen greases. Table 1 shows theresults.

<Low-Temperature Fretting Test>

In conformity to ASTM D4170, a performance evaluation test was conductedby using a Fafnir fretting wear testing machine. Using 51204J as abearing, the test was conducted for eight hours in conditions where themaximum contact surface pressure was 2.0 GPa, the oscillation cycle was30 Hz, the oscillation angle was 12°, and the atmosphere temperature was−20°. The performance of each of the specimen greases was evaluated onthe basis of the wear amount (mg) per bearing.

TABLE 1 Example A-1 A-2 A-3 A-4 A-5 A-6 Base grease Base oil MineralMineral Mineral Mineral Mineral Mineral oil oil oil oil oil oilThickener Aromatic Aromatic Aromatic Aromatic Aromatic Aromatic ureaurea urea urea urea urea Additive (part by weight¹⁾) Phosphite esterDimethyl phosphite 2 — — — 0.5 4 Diethyl phosphite — 2 — — — — Trimethylphosphite — — 2 — — — Triethyl phosphite — — — 2 — — Phosphate esterTributyl phosphate — — — — — — Triphenyl phosphate — — — — — — TCP(tricresyl phosphate) — — — — — — Kinetic viscosity of base oil 68 68 6868 68 68 (40° C. mm²/s) Worked penetration (JIS K2220) 280 260 260 280260 260 Low-temperature fretting test, mg 1 3 3 3 1 3 Comparativeexample A-1 A-2 A-3 A-4 A-5 A-6 Base grease Base oil Mineral MineralMineral Mineral Mineral Mineral oil oil oil oil oil oil ThickenerAromatic Aromatic Aromatic Aromatic Aromatic Aromatic urea urea ureaurea urea urea Additive (part by weight¹⁾) Phosphite ester Dimethylphosphite — — — — — — Diethyl phosphite — — — — — — Trimethyl phosphite— — — — — — Triethyl phosphite — — — — — — Phosphate ester Tributylphosphate — 2 — — — — Triphenyl phosphate — — 2 — — — TCP (tricresylphosphate) — — — 2 1 4 Kinetic viscosity of base oil 68 68 68 68 68 68(40° C. mm²/s) Worked penetration (JIS K2220) 280 280 300 280 280 280Low-temperature fretting test, mg 11 12 7 5 5 6 ¹⁾Part by weight for 100parts by weight of base grease

As shown in the specimen grease of the comparative example A-1 in whichthe additive was not added to the base grease, the specimen grease wasinferior in the fretting wear resistance thereof at low temperatures. Asshown in the specimen grease of each of the comparative examples A-2through A-4 in which the additive other than the phosphite ester used inthe present invention was added to the base grease, each of the specimengreases was inferior in the fretting wear resistance thereof at lowtemperatures. As shown in each of the specimen grease of each of thecomparative examples A-5 and A-6 in which the additive other than thephosphite ester used in the present invention was added to the basegrease in a smaller amount and in a larger amount respectively, each ofthe specimen greases was inferior in the fretting wear resistancethereof at low temperatures. On the other hand, the specimen grease ofeach of the examples A-1 through A-6 was excellent in the fretting wearresistance thereof at low temperatures.

Examples B-1 Through B-12 and Comparative Examples B-1 Through B-4

The specimen grease of each of the examples and the comparative exampleswas prepared to use them for a bearing of a motor. As shown in tables 2through 4, the base grease of each of the specimen greases of theexamples and the comparative examples was prepared by selecting athickener and base oil. Regarding the composition of each of the basegreases, the total of the amount of the thickener and that of the baseoil was 100 parts by weight. “Lithium soap”, shown in the tables, whichis the thickener was 12-lithium hydroxystearate. Each grease containing“aliphatic urea” as its thickener was obtained by dissolving the MDI(MILLIONATE MT (main component: diphenylmethane diisocyanate) producedby Nippon Polyurethane Industry Co., Ltd.) in the half amount of thebase oil shown in each table and octylamine whose equivalent amount wastwice as large as that of the MDI in the remaining half of the base oiland mixing these substances with each other with these substances beingstirred.

“Ester oil” of each of the examples B-1 through B-6 and the comparativeexamples B-1 and B-2 shown in the tables was mixed oil obtained bymixing an ester oil 1 (HATCOL H3110 produced by Nippon Steel ChemicalCo., kinetic viscosity at 40° C.: 12 mm²/s (diester oil)) and an esteroil 2 (ADEKALUBE 60Z01A produced by ADEKA CORPORATION, kinetic viscosityat 40° C.: 32 mm²/s (polyol ester oil)) with each other by setting aratio (ratio by weight) between the ester oil 1 and the ester oil 2 to3:7.

“PAO oil” of each of the examples B-7 through B-12 and the comparativeexamples B-3 and B-4 shown in the tables was SYNFLUID 601 (kineticviscosity at 40° C.: 30 mm²/s) produced by Nippon Steel Chemical Co.“Ester oil” of each of the examples B-7 through B-12 and the comparativeexamples B-3 and B-4 was HATCOL 2362 (kinetic viscosity at 40° C.: 72mm²/s (polyol ester oil)) produced by Nippon Steel Chemical Co. Base oilof each of the examples B-7 through B-12 and the comparative examplesB-3 and B-4 was mixed oil obtained by mixing the ester oil and the PAOwith each other by setting a ratio (ratio by weight) between the esteroil 1 and the PAO to 1:3.

After an additive was added to 100 parts by weight of each of the basegreases prepared in the above-described manner at a mixing ratio shownin tables 2 through 4 respectively, each of the mixtures wassufficiently stirred by the stirring/defoaming machine to obtain aspecimen grease of each of the examples and the comparative examples. 1)and 2) shown in each of tables 3 and 4 are the same as 1) and 2) shownat the bottom of table 2. A fretting test shown below was conducted onthe obtained specimen greases to measure the wear amount of each of thespecimen greases. Tables 2 through 4 show the results.

<Fretting Test>

In conformity to ASTM D4170, a performance evaluation test was conductedby using the Fafnir fretting wear testing machine. Using 51204J as abearing, the test was conducted for two hours in conditions where themaximum contact surface pressure was 1.7 GPa, the oscillation cycle was30 Hz, the oscillation angle was 12°, and the atmosphere roomtemperature was 20° C. The performance of each of the specimen greaseswas evaluated on the basis of the wear amount (mg) per bearing.

TABLE 2 Example B-1 B-2 B-3 B-4 B-5 B-6 Base grease Base oil Ester EsterEster Ester Ester Ester oil oil oil oil oil oil Thickener LithiumLithium Lithium Lithium Lithium Lithium soap soap soap soap soap soapAmount of thickener 11 11 11 11 11 11 (part by weight ¹⁾) Additive (partby weight²⁾) Dimethyl phosphite 2 — — — 0.5 4 Diethyl phosphite — 2 — —— — Trimethyl phosphite — — 2 — — — Triethyl phosphite — — — 2 — — TCP(tricresyl phosphate) — — — — — — Kinetic viscosity of base oil 26 26 2626 26 26 (40° C. mm²/s) Worked penetration (JIS K2220) 244 244 244 244244 244 Fretting test, mg 2 2 2 2 2 2 ¹⁾ Content (part by weight) ofthickener which occupies in 100 parts by weight of base grease (baseoil + thickener) ²⁾Part by weight for 100 parts by weight of base grease

TABLE 3 Example B-7 B-8 B-9 B-10 B-11 B-12 Base grease Base oil PAOoil + PAO oil + PAO oil + PAO oil + PAO oil + PAO oil + ester oil esteroil ester oil ester oil ester oil ester oil Thickener AliphaticAliphatic Aliphatic Aliphatic Aliphatic Aliphatic urea urea urea ureaurea urea Amount of thickener 12 12 12 12 12 12 (part by weight ¹⁾)Additive (part by weight²⁾) Dimethyl phosphite 2 — — — 0.5 4 Diethylphosphite — 2 — — — — Trimethyl phosphite — — 2 — — — Triethyl phosphite— — — 2 — — TCP (tricresyl phosphate) — — — — — — Kinetic viscosity ofbase oil 40 40 40 40 40 40 (40° C. mm²/s) Worked penetration (JIS K2220)243 243 243 243 243 243 Fretting test, mg 3 3 3 3 3 3 ¹⁾ Content (partby weight) of thickener which occupies in 100 parts by weight of basegrease (base oil + thickener) ²⁾Part by weight for 100 parts by weightof base grease

TABLE 4 Comparative example B-1 B-2 B-3 B-4 Base grease Base oil Esteroil Ester oil PAO oil + PAO oil + ester oil ester oil Thickener LithiumLithium Aliphatic Aliphatic soap soap urea urea Amount of thickener 1111 12 12 (part by weight¹⁾) Additive (part by weight²⁾) Dimethylphosphite — — — — Diethyl phosphite — — — — Trimethyl phosphite — — — —Triethyl phosphite — — — — TCP (tricresyl phosphate) — 2 — 2 Kineticviscosity of base oil 26 26 40 40 (40° C. mm²/s) Worked penetration 244244 243 243 (JIS K2220) Fretting test, mg 4 4 5 5 ¹⁾Content (part byweight) of thickener which occupies in 100 parts by weight of basegrease (base oil + thickener) ²⁾Part by weight for 100 parts by weightof base grease

As shown in the specimen greases of the comparative examples B-1 and B-3in which no additive was added to the base grease, the specimen greaseswere inferior in the fretting wear resistance thereof. As shown in thespecimen grease of each of the comparative examples B-2 and B-4 in whichthe additive (TCP) other than the phosphite ester used in the presentinvention was added to the base grease, the specimen greases had thesame effect as that of the specimen greases in which the TCP was notadded to the base grease. Thus the fretting wear resistance of thesespecimen greases could not be improved. On the other hand, the specimengrease of each of the examples B-1 through B-12 was excellent in thefretting wear resistance thereof.

Examples C-1 Through C-12 and Comparative Examples C-1 Through C-4

The base grease of each of the examples and the comparative examples wasprepared to use them for a bearing of a wind power generator. As shownin tables 5 through 7, the base grease of each of the examples and thecomparative examples was prepared by selecting a thickener and base oil.Regarding the composition of each of the base greases, the total of theamount of the thickener and that of the base oil was 100 parts byweight. “Synthetic oil”, shown in the tables 5 through 7, which is thebase oil was Mobilgear SHC XMP460 (trade name) produced by Exxon MobilCorporation.

“Lithium complex” (lithium complex soap) which is a thickener wasobtained by adding lithium hydroxystearate to the base oil, saponifyinga predetermined amount of the lithium hydroxystearate at a rate of onemole of lithium hydroxide monohydrate for one mole of the hydroxystearicacid, adding azelaic acid to the solution and saponifying the azelaicacid at a rate of one mole of the lithium hydroxide monohydrate for onemole of the azelaic acid, heating the solution up to 200° C., coolingthe solution, and mixing the above-described components with each otherby using a triple roll mill.

“Urea” which is the thickener was obtained by dissolving the MDI(MILLIONATE MT (main component: diphenylmethane diisocyanate) producedby Nippon Polyurethane Industry Co., Ltd.) in the half amount of thebase oil and the p-toluidine whose equivalent amount was twice as largeas that of the MDI in the remaining half of the base oil and mixingthese substances with each other with these substances being stirred.

After an additive was added to 100 parts by weight of each of the basegreases prepared in the above-described manner at a mixing ratio shownin tables 5 through 7 respectively, each of the mixtures wassufficiently stirred by the stirring/defoaming machine to obtain aspecimen grease of each of the examples and the comparative examples. 1)and 2) shown in each of tables 6 and 7 are the same as 1) and 2) shownat the bottom of table 5. A fretting test shown below was conducted onthe obtained specimen greases to measure the wear amount of each of thespecimen greases. Tables 5 through 7 show the results.

<Fretting Test>

In conformity to ASTM D4170, a performance evaluation test was conductedby using the Fafnir fretting wear testing machine. Using 51204J as abearing, the test was conducted for two hours in conditions where themaximum contact surface pressure was 1.7 GPa, the oscillation cycle was30 Hz, the oscillation angle was 12°, and the atmosphere roomtemperature was 25° C. The performance of each of the specimen greaseswas evaluated on the basis of the wear amount (mg) per bearing.

TABLE 5 Example C-1 C-2 C-3 C-4 C-5 C-6 Base grease Base oil SyntheticSynthetic Synthetic Synthetic Synthetic Synthetic oil oil oil oil oiloil Thickener Lithium Lithium Lithium Lithium Lithium Lithium complexcomplex complex complex complex complex Amount of thickener 17 17 17 1717 17 (part by weight ¹⁾) Additive (part by weight²⁾) Dimethyl phosphite2 — — — 0.5 4 Diethyl phosphite — 2 — — — — Trimethyl phosphite — — 2 —— — Triethyl phosphite — — — 2 — — TCP (tricresyl phosphate) — — — — — —Kinetic viscosity of base oil 460 460 460 460 460 460 (40° C. mm²/s)Worked penetration (JIS K2220) 300 300 300 300 300 300 Fretting test, mg3 3 3 3 5 5 ¹⁾ Content (part by weight) of thickener which occupies in100 parts by weight of base grease (base oil + thickener) ²⁾Part byweight for 100 parts by weight of base grease

TABLE 6 Example C-7 C-8 C-9 C-10 C-11 C-12 Base grease Base oilSynthetic Synthetic Synthetic Synthetic Synthetic Synthetic oil oil oiloil oil oil Thickener Urea Urea Urea Urea Urea Urea Amount of thickener11 11 11 11 11 11 (part by weight ¹⁾) Additive (part by weight²⁾)Dimethyl phosphite 2 — — — 0.5 4 Diethyl phosphite — 2 — — — — Trimethylphosphite — — 2 — — — Triethyl phosphite — — — 2 — — TCP (tricresylphosphate) — — — — — — Kinetic viscosity of base oil 460 460 460 460 460460 (40° C. mm²/s) Worked penetration (JIS K2220) 300 300 300 300 300300 Fretting test, mg 3 3 3 3 5 5 ¹⁾ Content (part by weight) ofthickener which occupies in 100 parts by weight of base grease (baseoil + thickener) ²⁾Part by weight for 100 parts by weight of base grease

TABLE 7 Comparative example C-1 C-2 C-3 C-4 Base grease Base oilSynthetic Synthetic Synthetic Synthetic oil oil oil oil ThickenerLithium Lithium Urea Urea complex complex Amount of thickener 17 17 1111 (part by weight¹⁾) Additive (part by weight²⁾) Dimethyl phosphite — —— — Diethyl phosphite — — — — Trimethyl phosphite — — — — Triethylphosphite — — — — TCP (tricresyl phosphate) — 2 — 2 Kinetic viscosity460 460 460 460 of base oil (40° C. mm²/s) Worked penetration 300 300300 300 (JIS K2220) Fretting test, mg 23 20 20 19 ¹⁾Content (part byweight) of thickener which occupies in 100 parts by weight of basegrease (base oil + thickener) ²⁾Part by weight for 100 parts by weightof base grease

As shown in the specimen greases of the comparative examples C-1 and C-3in which no additive was added to the base grease, the specimen greaseswere inferior in the fretting wear resistance thereof. As shown in thespecimen grease of each of the comparative examples C-2 and C-4 in whichthe additive (TCP) other than the phosphite ester used in the presentinvention was added to the base grease, the specimen greases had thesame effect as that of the specimen greases in which the TCP was notadded to the base grease. Thus the fretting wear resistance of thesespecimen greases could not be improved. On the other hand, the specimengrease of each of the examples C-1 through C-12 was excellent in thefretting wear resistance thereof.

INDUSTRIAL APPLICABILITY

The grease composition of the present invention has an excellentfretting wear resistance in a wide temperature region from low to hightemperatures and particularly at low temperatures. Therefore it ispossible to preferably utilize the grease composition of the presentinvention as grease for various mechanical parts such as parts aimed atrestraining a relative motion of a fit-in portion of a shaft, a boltedportion, a riveted portion, a tapered coupler, and the like; and partssuch as a rolling bearing, a sliding bearing, a ball bush, a splinebearing, a flexible shaft coupling, a universal joint, a constantvelocity joint, a laminated leaf spring, a coil spring, an electriccontact point, a valve, a valve seat, a wire rope, and the like whichhave a slight reciprocating motion. It is possible to preferably utilizethe rolling bearing of the present invention as (1) the hub bearing, (2)the rolling bearing for supporting rotors of motors for industrialmachines and electric equipments and rotors of motors for drivingelectric cars, and (3) the rolling bearing for supporting the mainshaft, blade, and yaw of a wind power generator.

EXPLANATION OF REFERENCE NUMERALS AND SYMBOLS

-   1: deep groove ball bearing-   2: inner ring-   3: outer ring-   4: rolling element-   5: cage-   6: sealing member-   7: grease composition-   8 a, 8 b: opening-   11: hub ring-   12: driving inner ring-   13: outer ring (outward member)-   14: rolling element-   15: inner ring (inward member)-   16: hub bearing-   17: sealing member-   18: sealing member-   21: rotating shaft-   22: winding-   23: rotor-   24: commutator-   25: brush holder-   26: brush-   27: end frame-   28: jacket-   29: stator-   31: wind power generator-   32: rotor blade-   33: rotor shaft-   34: nacelle-   35: bearing (rolling bearing)-   36: speed-up gear-   37: generator-   38: tower-   39: motor-   40: reducer-   41: inner ring-   42: outer ring-   43: rolling element-   44: cage-   45: bearing housing-   46: seal-   47: yaw pivotal seat bearing

1-14. (canceled)
 15. A rolling bearing, which supports a rotor of amotor, said rolling bearing comprising an inner ring; an outer ring; aplurality of rolling elements disposed between said inner ring and saidouter ring; a sealing member provided at openings disposed at both axialends of said inner ring and said outer ring; and a grease compositionbeing in circumferences of said rolling elements, wherein said greasecomposition is a grease composition containing base oil, a thickener,and an additive, wherein said base oil has a kinematic viscosity at 40°C. of 20 to 50 mm²/s, wherein said additive contains at least one kindselected from the group consisting of a phosphite ester of chemicalformula (1) and a phosphite ester of chemical formula (2); wherein acontent of said phosphite ester is 0.5 to 4 parts by weight for 100parts by weight which is a total of an amount of said base oil and thatof said thickener; said phosphite ester is dimethyl phosphite, diethylphosphite, trimethyl phosphite or triethyl phosphite; and said additivecontains neither phosphite esters other than said phosphite ester shownby said formula (1) or by said formula (2) nor phosphate esters havingorganic groups whose carbon number is not less than three:

in chemical formulas (1) and (2), R₁, R₂, R₃, R₄, and R₅ denote a methylgroup or an ethyl group; R₁ and R₂ are the same group or differentgroups; R₃, R₄, and R₅ are the same group or different groups.
 16. Arolling bearing according to claim 15, wherein said base oil is at leastone selected from among ester oil and poly-α-olefin oil.
 17. A rollingbearing according to claim 16, wherein said base oil is said ester oilor a mixed oil of said ester oil and said poly-α-olefin oil.
 18. Arolling bearing according to claim 15, wherein a content of saidthickener which occupies in 100 parts by weight of a total amount ofsaid base oil and said thickener is 5 to 30 parts by weight.
 19. Arolling bearing according to claim 15, wherein said thickener is atleast one selected from among lithium soap and a urea compound.
 20. Arolling bearing according to claim 15, which is a deep groove ballbearing.
 21. A rolling bearing according to claim 15, which supportsrotors of motors for industrial machines and electric equipments androtors of motors for driving electric cars.
 22. A rolling bearing,comprising an inner ring; an outer ring; a plurality of rolling elementsinterposed between said inner ring and said outer ring; and a greasecomposition being packed in circumferences of said rolling elements,wherein said grease composition is a grease composition containing baseoil, a thickener, and an additive, wherein said additive contains atleast one kind selected from the group consisting of a phosphite esterof chemical formula (1) and a phosphite ester of chemical formula (2);wherein a content of said phosphite ester is 0.5 to 4 parts by weightfor 100 parts by weight which is a total of an amount of said base oiland that of said thickener; said phosphite ester is dimethyl phosphite,diethyl phosphite, trimethyl phosphite or triethyl phosphite; and saidadditive contains neither phosphite esters other than said phosphiteester shown by said formula (1) or by said formula (2) nor phosphateesters having organic groups whose carbon number is not less than three:

In chemical formulas (1) and (2), R₁, R₂, R₃, R₄, and R₅ denote a methylgroup or an ethyl group; R₁ and R₂ are the same group or differentgroups; R₃, R₄, and R₅ are the same group or different groups.
 23. Arolling bearing according to claim 22, wherein said rolling bearing isused as a hub bearing.
 24. A rolling bearing according to claim 23,wherein carbon steel for machine structural use is used as a material insaid hub bearing.
 25. A rolling bearing according to claim 23, whereinsaid thickener is a urea compound shown by a chemical formula (3):

in chemical formula (3), R₇ denotes a divalent hydrocarbon group whosecarbon number is 6 to 15, each of R₆ and R₈ denotes a monovalentaromatic hydrocarbon group and R₆ and R₈ are the same or different. 26.A rolling bearing according to claim 23, wherein said base oil is amineral oil, and a kinematic viscosity of said base oil at 40° C. is 20to 150 mm²/s.
 27. A rolling bearing according to claim 23, wherein acontent of said thickener which occupies in 100 parts by weight of atotal amount of said base oil and said thickener is 5 to 30 parts byweight.
 28. A rolling bearing according to claim 23, wherein a workedpenetration of said grease composition is 200 to 350.